A variety of human ailments owe their origin to pathogenic microorganisms, which include bacteria, virus and fungi. The presence of such pathogenic microorganisms lead to septicaemia, serious infections of upper and lower respiratory tract, CNS, meningitis, intra-abdominal tissue including peritoneum, genito-urinary tract, skin, and soft tissue, and a variety of other infections like systemic mycosis, candidiasis including infections caused by dermatophytes. During last 100 years, significant progress has been made to combat the diseases caused by such a large family of microbes with innumerable therapeutic agents of diverse chemical and biological nature that have become available as a short and long term cure. Such antimicrobials include aminoglycosides, penicillins, cephalosporins, macrolides, glycopeptides, fluoroquinolones, tetracycline, first and second line anti-TB drugs, anti-leprosy, anti-virals, polyene, triazole and imidazole anti-fungals, combinations like pyrimidine derivatives and trimethoprim and sulphamethoxizole.
The constant use of antibiotics in the hospital environment has selected bacterial populations that are resistant to many antibiotics. These populations include opportunistic pathogens that may not be strongly virulent but that are intrinsically resistant to a number of antibiotics. Such bacteria often infect debilitated or immunocompromised patients. The emerging resistant populations also include strains of bacterial species that are well known pathogens, which previously were susceptible to antibiotics. The newly acquired resistance is generally due to DNA mutations, or to resistance plasmids (R plasmids) or resistance-conferring transposons transferred from another organism. Infections by either type of bacterial population, naturally resistant opportunistic pathogens or antibiotic-resistant pathogenic bacteria, are difficult to treat with current antibiotics. New antibiotic molecules which can override the mechanisms of resistance are needed.
Over the years bacteria have developed several different mechanisms to overcome the action of antibiotics. These mechanisms of resistance can be specific for a molecule or a family of antibiotics, or can be non-specific and be involved in resistance to unrelated antibiotics. Specific mechanisms include degradation of the drug, inactivation of the drug by enzymatic modification, and alteration of the drug target (B. G. Spratt, Science 264:388 (1994)). There are, however, more general mechanisms of drug resistance, in which access of the antibiotic to the target is prevented or reduced by decreasing the transport of the antibiotic into the cell or by increasing the efflux of the drug from the cell to the outside medium. Both mechanisms can lower the concentration of drug at the target site and allow bacterial survival in the presence of one or more antibiotics which would otherwise inhibit or kill the bacterial cells. Some bacteria utilize both mechanisms, combining a low permeability of the cell wall (including membranes) with an active efflux of antibiotics. (H. Nikaido, Science 264:382-388 (1994)).
Decreasing the permeability of the outer membrane, by reducing either the number of porins or by reducing the number of a certain porin species, can decrease the susceptibility of a strain to a wide range of antibiotics due to the decreased rate of entry of the antibiotics into the cells. However, for most antibiotics, the half-equilibration times are sufficiently short that the antibiotic could exert its effect unless another mechanism is present. Efflux pumps are an example of such other mechanism. Once in the cytoplasm or periplasm a drug can be transported back to the outer medium. This transport is mediated by efflux pumps, which are constituted of proteins. Different pumps can efflux specifically a drug or group of drugs, such as the NorA system that transports quinolones, or Tet A that transports tetracyclines, or they can efflux a large variety of molecules, such as certain efflux pumps of Pseudomonas aeruginosa. In general, efflux pumps have a cytoplasmic component and energy is required to transport molecules out of the cell. Some efflux pumps have a second cytoplasmic membrane protein that extends into the periplasm.
The Multicomponent efflux pumps, belonging mainly to the resistance-modulation-division (RND) family members, found mostly in gram-negative bacteria, include the MDR pumps AcrAB-TolC and MexAB-OprM from E. coli and Pseudomonas aeruginosa. Interplay between efflux pumps may provide either additive or multiplicative effects on drug resistance (A. Lee et al., J. of Bacteriology, 2000, 182: 3142). MexAB-OprM, MexCD-OprJ, MexEF-OprN, MexXY-OprM, AcrAB-TolC, AcrEF, MarA, SoxS, and/or Tet pump/s are known to be present in Gram negative organisms such as P. aeruginosa and E. coli and are reviewed in recent publications and papers, such as Webber and Piddock, J. of Antimicrobial Chemother, 2003, 51: 39-11; Bambeke et. al J. of Antimicrobial Chemother, 2003, 51: 1055-1056, 74; Xian Zhi Li et. al., Journal of Antimicrob. Chemother., 2000, 45: 433 436; O Lomovskaya, et. al., Antimicrob. Agents and Chemother., 1999, 43: 1340 1346.
A biofilm is a structured group of microorganisms encapsulated within a self-developed polymeric extracellular matrix. Biofilms are typically adhered to a living or inert surface. In the human or animal body biofilms can form on any internal or external surface. Biofilms have been found to be involved in a wide variety of microbial infections in the body and, therefore, cause a number of conditions including urinary tract infections, middle-ear infections, formation of dental plaque and gingivitis.
Microorganisms present in a biofilm have significantly different properties from free-floating microorganisms of the same species. This is because the polymeric extracellular matrix acts to protect the microorganisms from the surrounding environment allows the microorganisms to cooperate and interact in various ways which are not exhibited by free-floating microorganisms. These complex communities of microorganisms present a unique challenge in that they are often resistant to classical means of antimicrobial control. Bacteria living in a biofilm exhibit increased resistance to antibiotics because the dense extracellular matrix and the outer layer of cells protect the interior of the biofilm from the effects of the antibiotics. Therefore, known antimicrobial agents will not have the same effect on bacteria present in a biofilm.
Thus, cellular factors affecting transport (both active and passive transport) of antibiotics (and antibacterial agents) into bacterial cells are important components of antibiotic resistance for many bacterial species. There exists a need to provide compounds and compositions that enhance the efficacy of antimicrobial agents, even when the efficacy of the antimicrobial may be affected adversely by antibiotic resistance.